Who is old Garnett? The current one? If so, than I think that team wins +50 games and contends for the title.
If old Garnett is worse than the current McGee, they win 45 games or so the first season and then it all depends on McGee's progression and development (I'm not optimistic) and on how good Brooks is (I've only watched him in 3 games I think and wasn't impressed, but maybe I caught his bad nights). If they become what they're supposed to become, then it's certainly a contender.